Reboot Pebble by simultaneously holding the Back and Select button for 10-seconds.

The reboot should automatically turn your Bluetooth on again, but please double check. Turn the Bluetooth on if it is not already.
(Launcher -> Settings -> Bluetooth)

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Launch the Pebble app .

Make sure that Pebble is properly paired and connected with your device For instructions click here).

Make sure that you are in an area that has a strong reception or wifi signal.

Try the update again. If it doesn't pop up automatically, you can trigger it from the Pebble Time mobile app. From the menu (three dots in the top right corner), select "Support", and then tap the "Update your Pebble" button to search for and download new firmware.

Please be sure to note any odd behavior so that you can give a detailed description if the update should fail again.

At what point did the update fail? Near the end? Mid-way? Didn't even start?

What were the respective screen messages that you received from your device and from Pebble when the update failed?

Is this the first time that you have attempted an update or have you successfully completed one in the past?